1. Field of the Invention
The present invention relates to a liquid crystal display driving method and a liquid crystal display driving apparatus, particularly to a driving method for driving a liquid crystal display provided with a display cell including a switching element, a pair of transparent electrodes arranged so as to face each other at a predetermined interval, and liquid crystal set between the pair of transparent electrodes and a driving apparatus to which the driving method can be applied.
2. Related Art
A liquid crystal display (LCD) has been known so far as a display unit for displaying an image such as a character or diagram among information processing units including a personal computer. In particular, an active matrix driven LCD constituted by arranging switching elements such as thin-film transistors (TFTs) like a matrix is able to securely control the density of picture elements and suited to display a quickly-moving dynamic image or a color image. Therefore, the LCD is considered to be prospective as a display unit to be substituted for a CRT. In the case of a TFT-type LCD, a plurality of sets of a TFT and a transparent electrode connected each other are formed like a matrix on one of a pair of transparent electrodes arranged so as to face each other and moreover, a plurality of gate lines for turning on a TFT for each string and a plurality of data lines for applying a voltage to liquid crystal through the turned-on TFT are arranged on it. Furthermore, a common electrode is formed on the whole surface of the other transparent electrode and the liquid crystal is sealed between the pair of transparent electrodes.
A TFT-type LCD driving circuit displays an image or the like on liquid crystal by applying a voltage to the gate lines to turn on TFTs for each string in order and applying a voltage (data voltage) with a level corresponding to the gradation of each picture element corresponding to a turned-on TFT string to the liquid crystal through each data line. When the TFTs are turned on, the light transmittance of the liquid crystal changes correspondingly to the level of the data voltage and electric charges are accumulated between electrodes. After the TFTs are turned off, the liquid crystal keeps the changed light transmittance by the accumulated electric charges. The service life of liquid crystal is shortened by continuously applying voltages with the same polarity to the liquid crystal. Therefore, the service life of the liquid crystal is lengthened by reversing the polarity of the data voltage every line or every frame to drive the liquid crystal according to the fact that the light transmittances of liquid crystal are equalized for applied voltages with the same absolute value even if the polarities of the voltage are different from each other.
As for a switching element of an TFT or the like, a parasitic capacitance C is present between a gate and a source as shown by a broken line in FIG. 11 and the potential of an electrode connected to the TFT is decreased by .DELTA.V due to the parasitic capacitance C when the TFT is turned off. Therefore, the absolute value of the inter-electrode voltage after the TFT is turned off decreases by .DELTA.V when the data voltage with the positive polarity (polarity for the TFT side to become positive) to the liquid crystal as shown in FIG. 12(A) and increases by .DELTA.V when the data voltage with the negative polarity (polarity for the TFT side to become negative). Resultingly, the absolute value of the inter-electrode voltage fluctuates. Therefore, the position (center) of 0 V of the amplitude of the data voltage is corrected by .DELTA.V and the inter-electrode voltage after the TFT is turned off is made constant independently of the polarity of the data voltage to prevent a screen from seizing due to flicker or deterioration of the liquid crystal.
Improvement of the so-called compatibility has strongly been requested for an LCD in recent years so that the LCD can be connected to various information processors and makes it possible to display an image in accordance with signals outputted from various information processors like a CRT. When an information processor displays an image on a display unit, image data signals for expressing the gradation of each picture element of an image to be displayed and timing signals such as a horizontal synchronizing signal and a vertical synchronizing signal for specifying display timing are outputted from the information processor. However, a horizontal scanning period (cycle of the horizontal synchronizing signal) or the like specified by the timing signals is not constant but it depends on the type of the information processor. An LCD driving circuit measures the timing of a gate-voltage applying time (on-time of a TFT) in accordance with the horizontal scanning period shown by an inputted timing signal. Therefore, when the horizontal scanning period changes, the on-time of the TFT changes and thus, a problem occurs.
That is, an TFT has a small electric-charge mobility (particularly, an a-Si TFT). Therefore, when the TFT is turned on, the current (charging current) flowing to an electrode from a data line through the TFT is relatively small and also, the change rate of an inter-electrode voltage is low. Moreover, the level of the charging current depends on the level of the gate voltage, source voltage, or drain voltage (data voltage) when turning on the TFT.
In general, a gate-on time is constant. Therefore, because the potential between a gate and a source gradually lowers in accordance with the movement of electric charges to liquid crystal when charging a data-side electrode to a positive polarity (see &lt;1&gt; in FIG. 11), the inclination of a change of the inter-electrode voltage (particularly, the inclination .alpha.1 at the end of charging: see FIG. 12) is small and the number of electric charges to be charged in a cell within a gate-on time decreases. However, when charging the data-side electrode to a negative polarity (see &lt;2&gt; in FIG. 11), the potential between the gate and the source gradually rises in accordance with the movement of electric charges to the liquid crystal. Therefore, the inclination of the change of the inter-electrode voltage (particularly, the inclination .alpha.2 at the end of charging: see FIG. 12) increases (.vertline..alpha.1.vertline.&lt;.vertline..alpha.2.vertline.) and the number of electric charges to be charged in the cell increases.
As described above, the change rate of the inter-electrode voltage is relatively small and the change rate is fluctuated due to the polarity of the data voltage. Therefore, for example, when the horizontal scanning period specified by a timing signal inputted from an information processor is shortened (the on-time of an TFT is shortened), the absolute value of the inter-electrode voltage decreases as a whole independently of the polarity of the data voltage because the TFT is turned off before the inter-electrode voltage changes sufficiently as shown in FIG. 12(B). Moreover, because the change rate of the inter-electrode voltage is low when applying the data voltage with the positive polarity, the absolute value (V.sub.NE) of the inter-electrode voltage further decreases. Therefore, the absolute value of the inter-electrode voltage changes correspondingly to the polarity of the data voltage and flicker, seizing of screen, or change of contrast occurs.
Moreover, when the horizontal scanning period is short compared to the change rate of the inter-electrode voltage, the so-called gate overlap scan (also known as double on pulse) may be performed in which data voltages with the same polarity are repeatedly applied to liquid crystal. The gate overlap scan is performed in order to simultaneously turn on each TFT of a display cell string after two lines to which data voltages with the same polarity are applied in driving a line when driving display cell strings connected to the same gate line by reversing the polarity of the data voltage to be applied to the strings every line and previously charge the display cell string after two lines. Thereby, the data voltage is applied to each display cell string half by half for two cycles of the horizontal scanning period in one-time screen scan. Therefore, it is possible to change the inter-electrode voltage to a voltage corresponding to the data voltage even when the change rate of the inter-electrode voltage is low.
By performing the above gate overlap scan, the period in which the data voltage is applied to each display cell increases twofold. When selectively performing the above gate overlap scan in accordance with the horizontal scanning period or the like in order to properly display an image by connecting an LCD to various information processors, the absolute value of the inter-electrode voltage fluctuates the same as the horizontal scanning period changes, and flicker, deterioration of liquid crystal, or change of contrast occurs even in accordance with whether to execute the gate overlap scan.